1. Field of the Invention
This invention relates generally to a printed circuit board ("PCB"), and more particularly, to impedance and trace size in PCBs.
2. Description of the Related Art
As chip size continues to be reduced and signals are clocked at ever increasing frequencies, high-speed, high-frequency signals being driven between chips via traces (channels) on a printed circuit board ("PCB") may be subject to detrimental impedance effects. One manifestation of these impedance effects is unwanted reflections due to impedance mismatches. The high-speed, high-frequency signals may also be prone to cross-talk and electromagnetic interference ("EMI"). To help reduce such effects, high-speed signal traces have been routed through the middle of the PCB in layers between power and ground planes. Layers may have to be switched and trace width changed. For example, a 6-layer PCB could be employed to bury the traces instead of a four-layer PCB. Routing traces in this manner could add a cost of $10-$30 per board due to the additional layers needed to provide route paths.
The size of the trace is another parameter affecting impedance, as manifested in, for example, the characteristic impedance of the trace. Characteristic impedance generally decreases with increasing trace surface area (or with trace width). For example, for an approximately 1.4 mil thick trace on a four-layer PCB to have a characteristic impedance as low as 25 ohms, the trace may have to be as wide as approximately 30 mils. Therefore, when, building a conventional four-layer PCB, it may not be possible to reduce characteristic impedance from, for example, 60-80 ohms down to 25 ohms using narrower traces. This may present a problem if a narrow trace having reduced trace characteristic impedance is required for a specific implementation. Moreover, a wide trace may use too much PCB area (real estate) in certain implementations.
Thus, there is a need to reduce, control, or tailor impedance in PCBs while also avoiding the use of overly wide traces. Such control could reduce cross-talk and lessen the effects of EMI as well.